There is still no satisfactory explanation for the low catalytic activity of tissue factor (TF)/factor VII(a) complexes towards coagulation factor X, as found on the apical surface side of cell layers. It has been hypothesized that TF exists in a latent form. Layers of cultured human smooth muscle cells, constitutively expressing TF, were immunogold-labeled for TF in situ and processed for electron microscopy. We showed that, besides internalization and accumulation in lysosomal-like structures, TF remained associated with noncoated, flask- shaped microinvaginations of the plasma membrane. These invaginations were identified as caveolae. In regions in which intercellular contacts were interrupted, more TF-positive caveolae were observed. Enzymatically detached smooth muscle cells exhibited a similar enlargement of caveolar structures. Concomitantly, an increase of catalytic activity of apically formed TF/VIIa complexes towards factor X was found on the suspended cells. We speculate that caveolae- associated TF may function as a latent pool of procoagulant activity, which can rapidly be activated at sites in which vessel wall integrity is lost.

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